The use of polymeric film bases for carrying photographic layers is well known. In particular, photographic elements which require accurate physical characteristics use polyester film bases, such as poly(ethylene terephthalate) film bases. In fact, polyester film bases, when compared with commonly used cellulose ester film bases, are dimensionally more stable and more resistant to mechanical stresses under most conditions of use.
The formation of static electric charges on the film base is a serious problem in the production of photographic elements. While coating the light-sensitive photographic emulsion, electric charges accumulated on the base discharge, producing light which may be recorded as an image on the light-sensitive layer. Other drawbacks which result from the accumulation of electric charges on polymeric film bases are the adherence of dust and dirt and coating defects.
Additionally, photographic elements comprising light-sensitive layers coated onto polymeric film bases, when used in rolls or reels which are mechanically wound and unwound or in sheets which are conveyed at high speed, tend to accumulate static charges and record the light generated by the static discharges.
The static-related damages may occur not only before the photographic element has been manufactured, exposed and processed, but also after processing when the photographic element including the image is used to reproduce or enlarge the image. Accordingly, it is desired to provide permanent antistatic protection, that is an antistatic protection which retains its effectiveness even after photographic processing.
Several techniques have been suggested to protect photographic elements from the adverse effects of static charges.
Matting agents, hygroscopic materials or electroconductive polymers have been proposed to prevent static buildup, each acting with a different mechanism. However, matting agents cause haze, dust and dirt problems, hygroscopic materials cause sheets or films to stick together or with other surfaces, and electroconductive polymers are not permanent after photographic processing or are not transparent when coated with conventional binders.
U.S. Pat. No. 4,225,665 purports to disclose permanent antistatic layers for photographic elements. Said layers consist essentially of three components: (1) a water-soluble, electrically conductive polymer comprising carboxylic groups, (2) a hydrophobic polymeric binder containing carboxylic groups, and (3) a polyfunctional aziridine crosslinking agent. This compositon, however, does not give clear coatings and causes premature reactions among the components prior to coating. U.S. Pat. No. 4,701,403 suggests a costly system of coating the components as two separate coatings to avoid these premature reactions.
U.S. Pat. No. 4,585,730 discloses a photographic element comprising a film base, a silver halide emulsion on one side of the support, and an antistatic layer on the other side of said support, wherein the antistatic layer is coated with an auxiliary gelatin layer containing a conductive polymer, whereby the antistatic properties of the antistatic layer are conducted through said auxiliary layer. Reference is made to U.S. Pat. Nos. 4,225,665 and 4,701,403 as useful antistatic layers to be coated with the auxilary layer according to U.S. Pat. No. 4,585,730. Said two layer construction, however, often suffers from poor adhesion between the two layers during photographic processing.
An antistatic layer coated onto a polymeric film base has been described in EP 486,982. That antistatic layer comprises the reaction product of a copolymer of the sodium polystyrene sulfonate and maleic acid (having a weight average molecular weight of 16,700) with a polyfunctional epoxide crosslinking agent. This antistatic layer provides good adhesion of photographic gelatin layers coated over it even during photographic processing. A problem with these antistatic layers is that the rate of crosslinking is low, and both drying and curing at high temperature is needed to have a permanent antistatic layer which is water resistant. Drying and curing at high temperature, however, may result in cracking of the layer itself. It is preferred, therefore, to dry the layer at low temperature, but the rate of crosslinking is consequently lowered and the time for preparing the antistatic base in the manufacture of photographic films is lenghthened.
As increased speed in manufacturing, conveying and processing a film is important in the photographic industry, improvement in permanent antistatcity and wet adhesion of photographic layers are strongly desired.